Servoed meter apparatus

ABSTRACT

Apparatus for positioning an indicator in accordance with a control signal including a D&#39;&#39;Arsonval-type meter movement having an auxiliary AC excitation winding on the field magnet thereby providing both a unidirectional and an alternating flux field for the meter movement magnetic circuit. The meter armature has two windings, one being excited by a control signal and reacting with the unidirectional field to displace the armature and the other operating to sense both the displacement of the armature and the rate of change of said displacement by reacting respectively with the alternating and unidirectional fields. When included in a closed-loop servosystem the meter movement operates as both a servodrive and an angular motion transducer providing displacement and rate feedback terms.

United States Patent inventor Charles E. Clift Phoenix, Ariz. Appl. No.815,933 Filed Apr. 14, 1969 Patented May 4, 1971 Assignee Sperry RandCorporation SERVOED METER APPARATUS 13 Claims, 4 Drawing Figs.

[1.8. CI 343/108, 318/584, 318/585, 318/650, 324/99, 340/27, 340/195Int. Cl G01r 17/06 Field of Search 343/108; 324/99, 146; 318/584, 585,650; 73/188; 340/195, 27 (NAV) References Cited FOREIGN PATENTS 194,9496/1967 U.S.S.R. 324/99 34 24 ff VOLTAGE a. 23

REGULATOR 31 a:

Primary Examiner-Rodney D. Bennett, Jr. Assistant Examiner-Richard E.Berger Attorney-S. C. Yeaton ABSTRACT: Apparatus for positioning anindicator in accordance with a control signal including a DArsonval-typemeter movement having an auxiliary AC excitation winding on the fieldmagnet thereby providing both a unidirectional and an alternating fluxfield for the meter movement magnetic circuit. The meter armature hastwo windings, one being 7 excited by a control signal and reacting withthe unidirectional.

MONITOR LEVEL DETECTOR AND FLAG DRIVER '7 6 1.. ROLL comm/mo AMP. "\.E;I 41 A'l'A'A' I 11/259 i 1' t F 7 BIAS}! ouToFj VIEW FILTER 45 I i 'I'44 =fl 45 FLIGHT DEMODULATOR omscron COMPUTER 43 I l i :IL/46 DRIVEPITCH METER CIRCUIT i COMMAND 76 I i FEEDBACK i i CIRCUIT J Po VALIDPATENTEI] HAY 41971 sum 2 2 -C. PICKOF TION ZZZ- 1 20 1a 35 COMMQFEEDBACK SIGNAL 70/? BYCHARLES 5 CL/FT CONTROL SIGNAL FEEDBACK SIGNALsanvoao METER APPARATUS BACKGROUND OF THE INVENTION dicator 'is providedby arranging and constructing the magnetic core and armature assembly ofa meter movement so that it operates not only as a drive memberfor theindicator, but also as an angular motion sensor providing both displacement and rate feedback signals. While the invention may be employed inmany different embodiments in which it is desired to provide accuratedisplacement of an indicator in ac- V cordance with a control signal, itwill be described herein in connection with the command pointers of anaircraft flight director instrument.

. 2. Description of the Prior Art The function and operation of a flightdirector in an aircraft control system is disclosed in detail in US.Pat. Nos. 2,6i3,350 and 2,613,352, assigned to the same assignee as thepresent invention. As disclosed in those patents, certain basic flightcontrol parameters are combined such that if the pilot controls theaircraft in roll and pitch in a manner to maintain the pointers of thedisplay instrument centered or zeroed, the aircraft will asymptoticallyapproach and thereafter maintain a predetermined flight path. Thecontrol signal referred to hereinafter may, for exemplary purposes,comprise a signal output of the computer shown in the above-mentionedpatents and the meter movement of the display instrument hereindisclosed may replace the pointer actuator mechanism depicted in thepatents. For purposes of illustrating the present inventive concept,only one of the command pointers and its controlling electronics isdisclosed in detail, it being understood that other pointer mechanismsmay be substantially identical thereto and indeed other pointers orindicators of a modern flight director indicator may be similarlycontrolled.

SUMMARY OF THE INVENTION In accordance with the teaching of the presentinvention, an indicator pointer is accurately displaced relative to areference or zero index in an accurate, high response, well dampedmanner by use of closed-loop or feedback servo techniques, the pointerdriver or meter movement of the invention providing not only the drivingforce for the pointer, but also both the displacement and rate feedbackterms required by a high performance closed loop system.

ln a preferred embodiment of the invention, a permanent magnet ispositioned relative to a horseshoe shaped magnetic core in a manner toestablish a uniform DC magnetic field across an airgap between the coreand magnet, the flux lines running through the core structure back intothe end of the magnet remote from the airgap. An AC excited coil woundaround the magnetic core establishes an alternating flux field in theairgap in substantially collinear alignment with the DC field. Aneccentrically pivoted armature assembly comprising control and pickoffcoils positioned to move in the airgap is driven by means of a commandsignal applied to the control coil. As the armature assembly rotates, avarying DC signal is produced in the pickoff coil by virtue of itswindings cutting through the DC flux field. This DC signal has anamplitude and polarity determined respectively by the rate and directionof the displacement of the armature assembly. At the same time, as thearmature rotates, the pickoff coil becomes coupled to the AC flux fieldand thereby has induced in it an AC signal having an amplitude and phasedependent on the magnitude and direction of displacement of thearmature. The DC and AC signals induced in the armature pickoff coilconstitute respective rate and displacement feedback signals which aredegeneratively combined with the command signal applied to the armaturecontrol coil.

The armature and pointer are normally biased by means of leaf springsconnected to the armature coil windings such that in a nonoperatingcondition the pointer is held off scale or out of view against a stopaffixed to the magnetic core structure. In operation then, theclosed-loop servo operation provided by the displacement and ratefeedback signals holds the pointer at a zero or center position in theabsence of a command signal being applied to the armature controlwinding. This feature of the apparatus is utilized as explained in thefollowing detailed description to detect various malfunctions that mayarise during the course of operation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of ameter mechanism and associated control circuits constructed inaccordance with the present invention and shown in relation to a flightdirector computer and indicator therefor;

FIG. 2a is a top plan view of the meter mechanism shown in FIG. 1',

FIG. 2b is a sectional view of the meter mechanism taken along a planepassing through the meter pointer pivot axis and the longitudinal axisof the permanent magnet affixed to the meter core structure; and

FIG. 3 is an exploded view in perspective form depicting the armatureassembly and leaf spring arrangement of the meter mechanism.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1, 2a and 2b,roll pointer 10 is positioned laterally relative to a zero referenceindex 36 in the face 37 of a flight director indicator by means of ameter movement 11 of the eccentrically pivoted DArsonval-type inaccordance with a roll command signal supplied from flight directorcomputer 12. The meter movement comprises 5 conventional magnetic corearrangement having a base portion 13 of magnetically permeable materialto which is secured a permanent magnet 14 having north and south polesas shown, the free or north end of the magnet 14 being provided with anarcuate pole piece 15. Integrally formed on meter base 13 are bearingsupport bridges 16 and 17 having respective conventional jewel and pivotbearing assemblies 18 and 19 that define the pivot axis 21 of the meter.The support end of pointer 10 is secured to a U-shaped balance crossarm20, the vertically spaced arms of which support the bearing pins 18' and19' of jewel bearing pivot assemblies 18 and 19. A horseshoe-shapedkeeper 22 of magnetically permeable material has its open arms securedin magnetically permeable relation to base 13 and its closed endarcuately shaped to conform to the pole face 15. Thus, pole face 15 andthe internal surface of keeper 22 define an arcuate shaped airgap 23having a radius of curvature referenced to the pivot axis 21. Likewise,surface 13' of base 13 also has a radius of curvature referenced to thepivot axis so as to provide freedom of movement between balance crossarm20 and armature assembly 38.

The armature assembly comprises an open rectangular frame or bobbin 24rigidly secured to a pivot frame comprising extension members 25 and 26connected to balance arm 20 so as to be rotatable about the pivot axisand having wound thereon for movement therewith two separate coils 27and 28 as shown in FIG. 3, the coils surrounding the magnet core suchthat convolutions of each lie in and move through the airgap as thepointer rotates about the pivot axis. In the zero or undeflectedposition of the armature, the coils are collinear with the longitudinalaxis of magnet 14.

Associated with each pivot bearing assembly are conventional hairsprings29 and 30 arranged to bias balance arm 20 and pointer 10 fully againstmeter stop 31 so that in the unenergized condition of armature winding27 the pointer is held out of view behind mask 39 of the flight directorindicator and thus cannot be seen by the pilot. Stop 31' limits motionof the pointer in the opposite direction. Springs 29 and 30 are alsoused to conduct electrical energy to armature coils 27 and 28respectively, as shown schematically in FIG. 3. The coils have a commonconnection 32 which may be brought out of the armature assembly by meansof a flex-lead or pigtail but is preferably connected through a thirdhairspring 33.

From the foregoing description, it will be evident that the core andmagnet structure of the meter movement 11 defines a relatively uniformunidirectional magnetic flux field 39.

which is substantially collinear with the axis of the permanent magnet.ln accordance with the present invention an additional alternatingmagnetic flux field is generated which is substantially coextensive withthe magnetic field produced by magnet 14. The alternating field flux isproduced 'by coil 34 wound around magnet 14 and excited from a suitablesource of accurately regulated 400 cycle alternating current suppliedfrom a precision voltage regulator 35 which is necessary to ensure agood linear and thermally stable feedback signal, as will be describedhereinafter.

in operation, meter movement ll positions the pointer is accordance withthe polarity and magnitude of a DC control signal supplied from flightdirector computer 12 to armature control coil 27 through interaction ofthe magnetic flux field provided by magnet 14. Rotation of the armatureassembly thereby couples annature pickofr" coil 28 to the alternatingmagnetic flux field produced by AC excitation winding 34. As a result analternating signal is induced in armature winding 28 having a phase andamplitude proportional to the direction and magnitude through which thearmature is displaced from its quiescent position of alignment with thelongitudinal axis of magnet 14. In addition, since the convolutions ofpickoff coil 28 in airgap 23 cut through the unidirectional fieldtherein, a DC signal component is also generated therein, the signalbeing proportional in polarity and amplitude to the direction andmagnitude of the rate of change of the displacement.

From the foregoing, it will be appreciated that the meter movementhereinabove descrimd may also be employed as a position transducer inwhich case the armature control coil 27 is either eliminated or leftunenergized and the armature frame 24 is mechanically connected to anydevice, the angular rate and position of which are to be detected forthe purpose of providing a composite signal proportional thereto.

When used in an aircraft flight director apparatus, meter movement 11positions pointer 10 in a closed-loop servo fashion in accordance with adisplacement command signal, such as a roll command signal from theflight director computer 12. This command signal is a DC signal suppliedfrom the remote computer 12 over suitable transmission leads 40 to thepanel-mounted flight director indicator. The signal is suitablyamplified in preamplifier 41 and drive or output amplifier 42 forapplication to control winding 27 of meter movement 1!. Displacement andrate feedback signal components are generated in winding 28, asdescribed above, and supplied to feedback amplifier 43, the output ofwhich is divided into two branches, a displacement branch 44 and a ratebranch 45 where the respective feedback signal components are detected.The alternating signal component proportional to pointer position isdemodulated in chopper demodulator 46 which is referenced to the same400 cycle supply as AC excitation winding 34, the resulting DC pulsesbeing smoothed by capacitor 47 and applied through scaling potentiometer48 to summing junction 49. The DC component of the feedback signal frompickoff winding 28 is supplied through coupling capacitor 50, filter 51and scaling potentiometer 52 to summing junction 4% where it is combinedwith the DC displacement signal. The rate channel is designed to pass adiscrete DC level or change of DC level within the dynamic frequencyrange of meter movement 11. Since the 400 cycle pickolf frequency isappreciably beyond this range, it is decoupled by filter 51 whereby theinstantaneous DC level is passed and the 400 cycle component is filteredto ground. Scaling potentiometers 48 and 52 are adjusted to provide thedesired displacement and rate response characteristics of pointer 10. ifdesired, potentiometers 48 and 52 may be replaced with fixed resistorssuitably selected to give the desired response and in practice this willusually be the case. The combined displacement and feedback signals aresummed i the output of amplifier 42 to a very low value, that is, withjust enough error signal to overcome the slight spring bias produced bythe hairsprings as described above.

By virtue of the fact that-meter movement 11 is positioned by aclosed-loop position servo, coupled with a spring bias which, in anunenergized condition maintains the pointer 10 out of view against ahardover stop, overall system monitoring may be readily accomplished.Most of the failures which are monitored by means of the presentinvention will result in a greater than normal signal at the output ofmeter drive amplifier 42. For detecting such larger than normal signalsa monitor level detector and warning flag driver circuit 61 is provided.This circuit is of conventional design comprising an output transistor62 which controls the energization of flag meter coil 63 of flag meter64. Flag meter 64 is normally spring biased so that when it isdeenergized warning flag 55 pops into view from behind mask 39. Powerterminal of the level detector is connected to a negative voltage supplywhich normally holds transistor 71 in anonconducting state so thattransistor 62 is conductive and the flag meter coil is connected throughthe collector to emitter terminals thereof to ground. When a signal ofeither polan'ty applied to level detector input terminal 72 from theoutput of drive amplifier 42 exceeds a predetermined threshold,transistor 62 becomes nonconducting and removes excitation from themeter flag coil. in the case of a positive signal exceeding thethreshold, transistor 71 is turned on and transistor 62 shutoff byvirtue of current flowing through diode 68. The same action occurs inthe case of a negative signal exceeding the threshold as a result ofconduction through Zener diode 65. This arrangement enables all theimportant causes of system failure to be monitored.

Continuity monitoring of transmission line 40 is accomplished byinducing a 400 cycle signal across the input load resistor 56 ofamplifier 41 by using a transformer 57 for DC isolation. Most flightdirector computers have a very low output impedance usually obtained byconnecting a large capacitance 58 across the computer output terminals.The secondary winding of transformer 57 is connected in series withresistance 59 and both are connected across resistor 56. Thus, if eitherinput lead 60 or 60' from the flight director opens, the 400 cyclesignal will no longer be shorted out but instead will be fully developedacross resistor 56. The presence of a large AC signal across resistor 56drives amplifier 41 into saturation causing level detector 61, which isconnected to sense the input to the meter control coil 27, to removeexcitation from the warning flag meter drive coil 63 thereby displayingflag 55 and at the same time biasing pointer 10 out of view. Thus, anopen circuit on transmission leads 40 can be discriminated from a normalzero command supplied by flight director computer 12.

Another failure readily monitored by the present system relates toeither the occurrence of mechanical interference between pointer 10 andother mechanical elements of the flight indicator or a condition whereinthe pointer becomes stuck to the stop which may occur due tocontamination of the stop elements. In the event either of thesemalfunctions occurs, the signal level at the output of drive amplifier42 will increase in response to an increase in either the commandvoltage of the feedback voltage and again actuate level detector 61 soas to pop the warning flag. The same operation and results will obtainif an open circuit develops in one or both of the meter armature coils27 and 28 or if the flight director computer fails. In the latterinstance, the flight director flag will pop as a result of the flightdirector valid signal on lead 66 in series with the flag meter coil 63and transistor 62 reducing to zero.

There are times during the operation of a flight director when the pilotmay desire for one reason or another to bias the flight director pointer10 out of view intentionally without displaying the waming flag. Mostflight director computers provide a large negative voltage for thispurpose, the voltage being supplied over the normal flight directoroutput leads. ln the present arrangement this bias out of view signalacts as a normal input to the system. The value of the bias signal mayvary depending upon the computer design but in any case is large enoughto drive the pointer into its stops. it has already been stated,however, that a signal great enough to drive the indicator into a stopwill result in a failure indication.- To preclude this, the bias signalis limited by Zener diode 67 connected in a feedback path aroundpreamplifier 41. Zener diode 67 prevents the bias signal from reachingits full value, the actual limit value thereof being just sufficient tohold the pointer out of view through normal servo operation but notagainst stop 31. Thus, the output of amplifier 42 is maintained at a lowvalue so that excitation is maintained on the flag meter coil.

Control of a pointer (not shown) driven by a pitch command signalfrom'the flight director computer is accomplished in the same manner bya separate unit 73 comprising a meter 74 and associated drive andfeedback circuits 75 and 76. The output of the drive circuit in thepitch control unit is connected to input terminal 69 of the leveldetector wherein Zener diode 65 and diode 68' respectively respond tonegative and positive pitch drive signals exceeding a prescribed levelfor the purpose of removing excitation from flag meter coil 63 so as tocause flag 55 to pop into view.

While the invention has been described in its preferred embodiment, itis to be understood that the words which have been used are words'ofdescription rather than limitation and that changes may be made withoutdeparting from the true scope and spirit of the inventionin its broaderaspects.

lclaim: 1

I. An electromagnetic transducer comprising:

a DArsonval-type meter structure having a magnetic core means arrangedto define a, unidirectional magnetic flux field,

means coupled with said core means for producing an alter natingmagnetic flux field substantially coextensive with said unidirectionalflux field, anarmature winding mountedjvfor movement relative to saidcore means and having convolutions thereof adapted to be displacedthrough said flux fields, and

means coupled with said winding for displacing the same whereby there isinduced said winding an alternating current signal component having aphase and amplitude proportional to the direction and magnitude of saiddisplacement through interaction with said alternating magnitude fluxand a direct current signal component of polarity and amplitudeproportional to the direction and magnitude of the rate of change ofsaid displacement through interaction with said unidirectional magneticflux.

2. The transducer as set forth in claim 1 wherein said windingdisplacing means comprises a further armature winding mounted integrallyand substantially coextensive with said first mentioned armature windingand means for supplying a direct current displacement control signal tosaid further winding whereby both said annature windings are displacedthrough interaction with said, unidirectional field in accordance withthe polarity and amplitude of said control signal.

3. Apparatus for positioning an indicator pointer in closedloop servofashion in accordance with the control signal comprising:

meter movement means of the D'Arsonval-type having a magnetic core meansfor producing a unidirectional magnetic flux field, means coupled withsaid core means for producing an alternating magnetic flux fieldsubstantially coextensive with said unidirectional magnetic flux field,

' and a pivotally mounted armature means including a pair of coilshaving convolutions thereof adapted to move in both of said flux fields,

means supplying a reversible polarity, variable magnitude DC controlsignal to one of said coils for displacing said armature through thereaction of said unidirectional magnetic field in accordance with thesense and magnitude of said control signal and thereby displacing saidother coil with respect to said alternating magnetic field whereby thereis induced therein an alternating current having a phase and amplitudeproportional to the direction and magnitude of said displacement throughreaction with said alternating magnetic field, and

means coupled with said other coil for supplying a servoloop feedbacksignal proportional to said armature displacement.

4. The apparatus as set forth in claim 3 wherein a DC signal is producedin said other coil proportional to the rate of change of saiddisplacement through reaction with said unidirectional field and furtherincluding means responsive to said DC component for supplying a furtherservoloop feedback signal proportional to the rate of movement of saidarmature,

5. Apparatus for positioning an indicator comprising: meter movementmeans of the DArsonval-type including magnetic core means for producinga unidirectional magnetic field, means coupled with said core means forproducing an alternating magnetic field substantially collinear withsaid unidirectional magnetic field, and pivotally mounted armature meanshaving first and second coils so arranged and constructed thatconvolutions of each are adapted to move across both of said magneticfields.

driver means-having input terminals for receiving a reversible polarity,variable magnitude DC command signal and an output terminal connected tothe first coil to provide a control signal thereto for displacing theartnature through reaction with the unidirectional magnetic field inaccordance with the magnitude and sense of the control signal wherebythe second coil is displaced with respect to the alternating magneticfield so that there is induced therein through reaction with thealternating magnetic ,field an AC feedback signal having a frequencycorresponding to that of the alternating magnetic field and an amplitudeand phase proportional to the magnitude and direction of saiddisplacement and through reaction with said unidirectional magneticfield a DC feedback signal having an amplitude and polarity proportionalto j the rate of change and direction of said displacement, and

feedback means coupling the second coil to the first coil such that theAC and DC feedback signals are degenera tively combined with the commandsignal.

6. The apparatus of claim 5 wherein the feedback means includes meansfor converting the AC feedback signal to a proportional DC convertedsignal having a polarity determined in accordance with the phase of theAC signal relative to the alternating magnetic field, and a filterconnected in parallel with the converter means, said filter operating topass the DC feedback signal and block the AC feedback signal.

7. The apparatus of claim 6 further including means for biasing thearmature means such that the indicator is held out of view against astop affixed to the meter movement in an unehergized state of the metermovement, and level detector means connected to the output terminal ofthe driver means for sensing a condition whereat the signal applied tothe first coil exceeds a predetemtined magnitude of either polarity.

8. The apparatus of claim 6 further including level detector meansconnected to the output terminal of the driver means for sensing acondition whereat the signal applied to the first coil exceeds apredetermined magnitude of either polarity.

9. The apparatus of claim 8 wherein the driver means includes means forlimiting its response to the DC command signal to preclude the indicatorfrom being driven against the sto 12). The apparatus of claim 8 furtherincluding a DC command signal source connected to the driver means, saidsource having a low AC output impedance, and means for coupling an ACfault detector signal to the input terminals of the driver means, saidAC fault detector signal being normally short-cin ing a flag metercomprising an actuator and warning flag coupled to the level detectormeans, the actuator being responsive to a signal from the level detectormeans for displaying the warning flag' when the control signal exceedssaid predetermined magnitude.

l3. The apparatus of claim 12 further including means connecting theflight director computer to the flag meter actuator for displaying thewarning flag upon occurrence of failure in the flight director computer.

1. An electromagnetic transducer comprising: a D''Arsonval-type meterstructure having a magNetic core means arranged to define aunidirectional magnetic flux field, means coupled with said core meansfor producing an alternating magnetic flux field substantiallycoextensive with said unidirectional flux field, an armature windingmounted for movement relative to said core means and having convolutionsthereof adapted to be displaced through said flux fields, and meanscoupled with said winding for displacing the same whereby there isinduced in said winding an alternating current signal component having aphase and amplitude proportional to the direction and magnitude of saiddisplacement through interaction with said alternating magnitude fluxand a direct current signal component of polarity and amplitudeproportional to the direction and magnitude of the rate of change ofsaid displacement through interaction with said unidirectional magneticflux.
 2. The transducer as set forth in claim 1 wherein said windingdisplacing means comprises a further armature winding mounted integrallyand substantially coextensive with said first mentioned armature windingand means for supplying a direct current displacement control signal tosaid further winding whereby both said armature windings are displacedthrough interaction with said unidirectional field in accordance withthe polarity and amplitude of said control signal.
 3. Apparatus forpositioning an indicator pointer in closed-loop servo fashion inaccordance with the control signal comprising: meter movement means ofthe D''Arsonval-type having a magnetic core means for producing aunidirectional magnetic flux field, means coupled with said core meansfor producing an alternating magnetic flux field substantiallycoextensive with said unidirectional magnetic flux field, and apivotally mounted armature means including a pair of coils havingconvolutions thereof adapted to move in both of said flux fields, meanssupplying a reversible polarity, variable magnitude DC control signal toone of said coils for displacing said armature through the reaction ofsaid unidirectional magnetic field in accordance with the sense andmagnitude of said control signal and thereby displacing said other coilwith respect to said alternating magnetic field whereby there is inducedtherein an alternating current having a phase and amplitude proportionalto the direction and magnitude of said displacement through reactionwith said alternating magnetic field, and means coupled with said othercoil for supplying a servoloop feedback signal proportional to saidarmature displacement.
 4. The apparatus as set forth in claim 3 whereina DC signal is produced in said other coil proportional to the rate ofchange of said displacement through reaction with said unidirectionalfield and further including means responsive to said DC component forsupplying a further servoloop feedback signal proportional to the rateof movement of said armature.
 5. Apparatus for positioning an indicatorcomprising: meter movement means of the D''Arsonval-type includingmagnetic core means for producing a unidirectional magnetic field, meanscoupled with said core means for producing an alternating magnetic fieldsubstantially collinear with said unidirectional magnetic field, andpivotally mounted armature means having first and second coils soarranged and constructed that convolutions of each are adapted to moveacross both of said magnetic fields, driver means having input terminalsfor receiving a reversible polarity, variable magnitude DC commandsignal and an output terminal connected to the first coil to provide acontrol signal thereto for displacing the armature through reaction withthe unidirectional magnetic field in accordance with the magnitude andsense of the control signal whereby the second coil is displaced withrespect to the alternating magnetic field so that there is inducedtherein through reaction with the alternating magnetic field an ACfeedback signal having a frequency correspondinG to that of thealternating magnetic field and an amplitude and phase proportional tothe magnitude and direction of said displacement and through reactionwith said unidirectional magnetic field a DC feedback signal having anamplitude and polarity proportional to the rate of change and directionof said displacement, and feedback means coupling the second coil to thefirst coil such that the AC and DC feedback signals are degenerativelycombined with the command signal.
 6. The apparatus of claim 5 whereinthe feedback means includes means for converting the AC feedback signalto a proportional DC converted signal having a polarity determined inaccordance with the phase of the AC signal relative to the alternatingmagnetic field, and a filter connected in parallel with the convertermeans, said filter operating to pass the DC feedback signal and blockthe AC feedback signal.
 7. The apparatus of claim 6 further includingmeans for biasing the armature means such that the indicator is held outof view against a stop affixed to the meter movement in an unenergizedstate of the meter movement, and level detector means connected to theoutput terminal of the driver means for sensing a condition whereat thesignal applied to the first coil exceeds a predetermined magnitude ofeither polarity.
 8. The apparatus of claim 6 further including leveldetector means connected to the output terminal of the driver means forsensing a condition whereat the signal applied to the first coil exceedsa predetermined magnitude of either polarity.
 9. The apparatus of claim8 wherein the driver means includes means for limiting its response tothe DC command signal to preclude the indicator from being drivenagainst the stop.
 10. The apparatus of claim 8 further including a DCcommand signal source connected to the driver means, said source havinga low AC output impedance, and means for coupling an AC fault detectorsignal to the input terminals of the driver means, said AC faultdetector signal being normally short-circuited by said low AC outputimpedance and effective to provide a signal at the output of the drivermeans in excess of said predetermined magnitude upon the occurrence ofan open circuit in the connection between the driver means and the DCcommand signal source.
 11. The apparatus of claim 10 wherein the drivermeans includes means for limiting its response to the DC command signal.12. The apparatus of claim 11 wherein the DC command signal source is aflight director computer and further including a flag meter comprisingan actuator and warning flag coupled to the level detector means, theactuator being responsive to a signal from the level detector means fordisplaying the warning flag when the control signal exceeds saidpredetermined magnitude.
 13. The apparatus of claim 12 further includingmeans connecting the flight director computer to the flag meter actuatorfor displaying the warning flag upon occurrence of failure in the flightdirector computer.